feat: 포인트 사용 로직 구현 (만료일 순 + 10포인트 단위 차감)

src/main/kotlin/kr/co/vividnext/sodalive/point/PointUsageService.kt

@@ -0,0 +1,43 @@
+package kr.co.vividnext.sodalive.point
+
+import org.springframework.stereotype.Service
+import java.time.LocalDateTime
+
+@Service
+class PointUsageService(
+    private val memberPointRepository: MemberPointRepository,
+    private val usePointRepository: UsePointRepository
+) {
+    fun usePoint(memberId: Long, contentPrice: Int): Int {
+        val now = LocalDateTime.now()
+        val maxUsablePoint = contentPrice * 10
+
+        val points = memberPointRepository.findByMemberIdAndExpiresAtAfterOrderByExpiresAtAsc(
+            memberId = memberId,
+            expiresAt = now
+        )
+
+        val totalAvailable = points.sumOf { it.point }
+        val usablePoint = minOf(totalAvailable, maxUsablePoint).floorToNearest10()
+
+        var remaining = usablePoint
+        var used = 0
+
+        for (p in points) {
+            if (remaining <= 0) break
+            val usable = minOf(p.point, remaining)
+            p.point -= usable
+            remaining -= usable
+            used += usable
+        }
+
+        if (used > 0) {
+            memberPointRepository.saveAll(points)
+            usePointRepository.save(UsePoint(memberId = memberId, amount = used))
+        }
+
+        return used
+    }
+
+    private fun Int.floorToNearest10(): Int = (this / 10) * 10
+}